The investigations are designed to (1) isolate and examine individual central neurons responsive to noxious stimuli, (2) evaluate the effects of centrally acting drugs on their responsiveness, and (3) examine the neuroanatomy and neurophysiological mechanism(s) of focal brain stimulation-induced analgesia. Glass micropipettes will be employed for purposes of extracellular unit recording in the cat trigeminal sensory complex, brainstem reticular core and thalamic para-and intra-laminar nuclei. Noxious stimuli will include electrical tooth pulp stimulation, electrical stimulation of the superficial radial and caudal cutaneous sural nerves, and radiant heat. The effects of chlorpromazine, pentobarbital, morphine and saline on unitary responses to these noxious stimuli will be compared to their effects on unitary response(s) to somatic non-noxious stimuli (e.g., light touch and pressure, tapping, etc.) for purposes of determining the significance of drug effects. The effects of these drugs (administered systemically) will be examined in unanesthesized, mechanically ventilated, decerebrate or paralyzed cats. Additional experiments will examine the neuroanatomy and neurophysiology of focal brain stimulation-induced analgesia. The objective of these investigations is to elaborate on the electrophysiologic mechanisms by which, and the neuroanatomical sites at which, analgesic drugs exert their effects to obtund pain. BIBLIOGRAPHIC REFERENCE: Gebhart, G.F. and Toleikis, J.R. Peri- $ aqueductal central gray focal brain stimulation-induced analgesia evaluated in cats. Fed. Proc. 34:435, 1975 (Abstract).